Given that the manufacturers of frames do not provide a screen (or additional face) of shape adapted to each frame put on the market, and in addition do not propose a screen of optical characteristics adapted to each requirement, the user of eyeglasses is faced with a very limited range of options: since the additional faces available on the market are of shapes that are generally not adapted to the shape of the frame of particular eyeglasses, the additional faces are mounted on the eyeglasses as an approximate fit and that generally gives rise to poor fixing; in addition, additional faces are generally unsuitable for being fixed to eyeglasses for children; a wearer of eyeglasses is thus faced with a restricted range of options for screens both in terms of shape and in terms of color.
U.S. Pat. No. 5,017,001 describes a monocular screen for eyeglasses including three fasteners of synthetic resin each in the form of an arch and each provided with a central projection; a front portion of the fastener is inserted in a groove provided at the periphery of the screen, and a rear portion of the fastener can clip onto the frame of the eyeglasses that receive the screen; the fastener is fixed to the glass of the screen either by adhesive applied to the inside face of the front portion of the fastener, or by making holes in the front portion of the fastener, in the central projection, and in the periphery of the lens; under such circumstances, the fastener is secured by means of a wire-like piece of material which is inserted through the orifices and which is held in place by adhesive or by being a force-fit, like a cotter or other pin.
The screen is assembled on the pair of eyeglasses by a transverse movement and by a pivoting movement; to this end, the three fasteners are disposed in a special configuration.
That monocular screen suffers from drawbacks: making grooves in the periphery of the screen lens requires special tooling such as a grinder, and also requires a high degree of accuracy in machining to obtain a groove of shape that matches that of the fastener; in addition, making grooves that open out into the periphery of a lens runs the risk of damaging the lens, in particular the risk of breaking it, specifically when the profile of the groove is angular in shape.
The monocular screen described in that document is difficult to fix to the frame of eyeglasses in stable manner because of the fasteners having special positions that are made necessary by the special movement for securing the monocular screen to the pair of eyeglasses, in particular because of the presence of a top fastener in a substantially central position and because of the presence of two bottom fasteners on the sides; that disposition also makes it necessary to identify accurately and measure accurately the height of the eyeglasses so as to match the positions of the fasteners as a function of the height of the eyeglasses, which requires a special measuring tool to be used; the special configuration of fasteners described in that document leads to the need to provide a fastener of matching shape for each thickness of screen lens in order to clamp the screen lens between the front portion of the fastener and its central projection; similarly, the screen described in that document requires a fastener of shape that is adapted to each thickness of frame or lens in the pair of eyeglasses that is to receive the screen, so as to clip onto the frame of the eyeglasses by means of the cavity provided between the lateral rear portion and the projection of the fastener.
In the embodiment where said fastener is secured to the lens of the monocular screen by a pin-forming wire-like piece, i.e. when said fastener is in two portions, this also requires precise orifices to be machined to enable the orifices to be brought into alignment and to allow the pin-forming wire-like piece to be put into place, and this also gives rise to special difficulties associated with handling a pin of very small size.
When the pin is glued, an additional difficultly lies in selecting an adhesive that is compatible with the three possibly different materials of the fastener, of the screen lens, and of the pin itself; under such circumstances, an additional problem results from the small size of the surfaces to be stuck together, which surfaces are also very difficult of access; an additional difficulty and drawback also results from the embodiment in which the pin-forming wire-like piece is a force-fit in orifices provided in the fastener and in the lens of the monocular screen, given the very small dimensions of such orifices and of the pin; in addition, that document gives no details about the materials and the tools required to obtain such a force-fit.
U.S. Pat. No. 5,123,724 describes a method of manufacturing a binocular screen for eyeglasses.